With an increase in the level of integration of semiconductor devices, progress has been made in the technology using X-rays as the light source for the exposure of wafers.
Japanese Kokai Patent Application No. Sho 63 1988!-311315 discloses a technology for the reduction exposure of wafers, with X-rays from the mask reflected by a first concave mirror, a convex mirror, and a second concave mirror, in that order. However, when using such a system in the synchronized scanning of a mask and wafers, the wafers interfere with the rays, which may lead to the rays being deflected.
The aforementioned Japanese patent also discloses a configuration for avoiding interference of the rays by the wafer during scanning by placing a plane mirror between the mask and the first concave mirror and/or a plane mirror between the second concave mirror and the wafer. However, with this configuration, since the incident angle of the X-rays on the plane mirror is about 45.degree., and the plane mirror for X-rays is formed as a laminated film, a phase shift takes place due to the difference in the incident angle, leading to aberration.
U.S. Pat. No. 5,315,629 discloses an optical system in which a first concave mirror, a plane mirror, a second concave mirror, and a third concave mirror are arranged coaxially, in that order, from the mask to the wafer. According to this system, it is possible to avoid interference of the rays by the wafer, but four aspherical mirrors are required. This is a disadvantage.
The purpose of this invention is to solve the problems of the aforementioned conventional systems by providing a reflective reduction imaging optical system for X-ray lithography that can provide a good imaging performance using a simple system.